1. Field of the Invention
This invention relates to pressure measuring apparatus, and more particularly to a diaphragm type gas operated pressure sensor which is quite small in size, is quick to provide an accurate pressure reading, and is capable of accurately sensing quite large pressures. The sensor has general utility but is particularly well suited for use in a piezometer system for the remote measuring of fluid pressures, such as hydrostatic pressure at various subsurface points in an earthen dam, embankments or in foundations.
2. Description of the Prior Art
Piezometers are known and have been used for many years in situations where it is important to know the stresses at locations which are not conveniently accessible including subsurface hydrostatic pressure. For example, it has long been known that the hydrostatic pressure within an earthen dam is important, and by knowing these pressures a prediction can be made as to whether the dam is performing according to its design specification or whether a fault has developed. Of course, once it is recognized that a fault has occurred at some location within the earthen dam, either immediate corrective measures can be implemented to repair the fault or, in the most extreme situation, advance warning can be provided for evacuation of those people who would be in the path of the torrent of water released by a rupture in the dam.
As is apparent, such pressure measuring systems necessarily involve the implantation of a sensing device in the ground at a distance which is far removed from the surface where the pressure measurements must be read. Direct pressure reading devices, such as gauges or the like, are both expensive and cannot practically be positioned so that they can accurately measure the pressure at the subsurface location, sometimes up to several thousand feet, and still be read at a convenient surface location.
Some prior art piezometers have been of the hydraulically actuated type wherein a remote sensor is positioned at the subsurface elevation, either during construction of a dam or by a drilling operation after construction, and one or more hydraulic fluid lines were provided which extended to the surface. These systems were often unsatisfactory because temperature variations often introduced inaccuracies into the pressure measurement as the result of the hydraulic fluid either freezing or considerably changing in viscosity and specific gravity during such temperature excursions. Additionally, hydraulic fluid, after prolonged exposure to subsurface conditions, tended to decompose creating byproducts which can be corrosive to the remote sensing unit or connecting lines.
Other piezometer systems have used electrically actuated component parts in which pressure fluctuations acting on the remote sensor creates a corresponding variable impedance to an electrical test signal so that an electrical output signal from the remote sensor is proportional to the ground pressure at the sensor location. The sensor is connected by an electrical circuit to a measuring instrument which transposes the electrical output signal into a readable form such as an indication on a meter or chart. These electric piezometers have proven to be somewhat undesirable as the result of their rather high cost, basically due to the expensive remote sensing unit, in addition to a low reliability and projected operating life as the result of the corrosive effect of the soil conditions and/or ground water on the remote sensor unit.
Still other piezometer systems have been of the gas type in which a remote sensor is disposed in the subsurface area of interest and is connected by a dual conduit line to the measuring station on the surface. Of particular interest with respect to this type of piezometer is U.S. Pat. No. 3,388,598, issued June 18, 1968 to Earl B. Hall. The piezometer system described in this patent includes a pressure sensing cell with a bearing wall into which two spaced-apart passageways open. A planar metal diaphragm is normally seated against the bearing wall by the ground pressure being sensed. The internal passages within the load cell are connected by an inlet and outlet conduit which lead to the surface. To measure the hydrostatic pressure at the subsurface location, a supply of gas, under pressure greater than the pressure to be sensed, is connected to the inlet conduit by a valve which varies both the flow rate and pressure of the supplied gas. A pressure gauge is also connected to the inlet conduit and a gas flow meter is connected to the outlet conduit. When gas from the high pressure supply passes through the inlet conduit to the diaphragm at a pressure sufficiently high to unseat the diaphragm by a predetermined distance, this separation will be reflected by a certain flow rate of gas at the flow meter on the surface. At this point the pressure gauge then indicates the pressure against the diaphragm, and hence, the subsurface hydrostatic ground pressure in the area of the remote sensor.
A particular problem of a remote sensing unit of the type disclosed by this patent is that the metal diaphragm must be constructed quite large in diameter to give it sufficient flexibility to enable it to move from a closed to an open position. A large diameter results in the diaphragm displacing a substantial amount of liquid when it does move. The liquid displaced by the diaphragm does not quickly flow back into the soil because of a low permeability condition of the soil. As a result, displacement of the large diameter diaphragm increases the external pressure acting on the diaphragm and this results in an inaccurate reading. In some soils it takes a substantial length of time for the pressure signal generated by the diaphragm displacement to dissipate so that only the hydrostatic ground pressure is being measured.
Prior to making the present invention, I attempted to solve the problems experienced by the prior pressure sensors by making the diaphragm out of an elastomeric material and in the shape of a cup. Initially, I constructed the cylindrical support member for such diaphragm, which was constructed to fit snugly inside of the diaphragm, to include an axial row of three radial inflow ports and a diametrically opposed axial row of three radial outflow ports. Such sensor proved to be unreliable. It resulted in erroneous readings that were sometimes positive and sometimes negative and the diaphragm would oscillate. I attempted to solve these problems by adding a single axial port leading from the inflow passageway towards the end of the diaphragm, but this did not solve the problems. The return rate for these sensors fell within the range of 55 to 80 percent, making a change in design of the sensor a necessity.